Bolt for orthopaedic surgical use
The present invention relates to a bolt for orthopaedic surgical use, which in particular may be used in combination with an external fixing device, such as a monoaxial extender. 
     Known bolts used in the elongation of limbs through the use of monoaxial extender devices generally have a cylindrical portion followed by a tapered portion terminating in a point having a varying degree of sharpness. In this case, the tapered portion is threaded and the thread may be a tapered spiral thread or a cylindrical spiral thread. There are, however, also bolts having a completely cylindrical section with a thread which is restricted to a portion close to the point, equal to about one third of the total length of the bolt. All the types of bolts described above have circular transverse cross-sections over almost the entire length of the bolt. Generally the only exception to this is the end opposite the point which has a short suitably shaped portion whose function is to permit the bolt to be turned by means of a suitable instrument. When a segment of bone is to be extended at least two bolts are applied to both sides of the point at which the bone which is to be extended is cut, and then the extender device is fitted to the heads of the bolts projecting from the patient&#39;s skin and extension then takes place. 
     Normally a bolt is of such dimensions that the threaded portion is almost completely inserted into the bone, and therefore the portion of the bolt which projects from the bone and which is in contact with the soft tissues and the patient&#39;s skin is a generally cylindrical or frustoconical portion having a smooth lateral surface. During the movements due to extension this portion of the bolt, which normally has a diameter of some 6 mm, compresses the soft tissues with which it is in contact. This compression often gives rise to the occurrence of cutaneous or subcutaneous necrosis close to the point stressed. 
     This disadvantage is most obvious in achondroplastic patients of the female sex who have a subcutaneous panniculus adiposus. 
     The excessive pressure applied can also give rise to retraction scars which are aesthetically and functionally unacceptable to the patient. 
     The object of the invention is to at least minimise the abovementioned disadvantages and in particular to provide a bolt which reduces compression of the surrounding soft tissues to a minimum while maintaining unaltered the required strength characteristics. 
     According to the present invention there is provided a bolt for orthopaedic surgical use, which can be combined with an external fixing device, said bolt comprising a substantially cylindrical upper portion having an upper end which can be used to tighten the bolt by means of a suitable instrument, and extending into a lower threaded portion which terminates at its end in a point in which a central portion is provided between said upper and lower portions, the lateral surface of said central portion having a cutting member which extends parallel to the longitudinal axis of the bolt over the entire length of the central portion, and in which the ends of the lateral surface of the said portion are graded into the lateral surfaces of the said portion are graded into the lateral surfaces of the upper portion and the lower portion. 
     The main advantage offered by the arrangement proposed lies in the fact that a bolt according to the invention avoids occurrence of the cutaneous or subcutaneous necrosis mentioned, thus reducing suffering by the patient. 
     Another advantage is due to the fact that full thickness incisions under local anaesthetic, which make it necessary for the patient to be admitted to hospital, are no longer necessary. 
     A further advantage is due to the fact that medical checks during the entire elongation process can be reduced appreciably, and can take place at less frequent intervals. 
     As a result of this the patient, who is normally in his own home, has to make less journeys from home to hospital and doctors who are less involved in such checks can thus dedicate their time to other patients. 
    
    
     The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a side view of a bolt according to the invention; 
     FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1 of a first embodiment of the invention; 
     FIG. 3 is a sectional view, similar to that of FIG. 2, of a second embodiment of the invention; and 
     FIG. 4 is a sectional view, similar to that of FIG. 2, of a third embodiment of the invention. 
     In the drawings, identical or equivalent parts have the same reference numerals. 
    
    
     As illustrated in FIG. 1, a bolt, of a substantially cylindrical shape, is constructed having an initial portion 10, a central portion 11 and a terminal portion 12. 
     Initial portion 10, which is perfectly cylindrical and of a length equal to about one third of the entire length of the bolt, has a short portion at upper end 14 in which the lateral surface is flattened in order to allow a suitable instrument to tighten up or slacken off the said bolt. 
     Terminal portion 12 is cylindrical, having an external diameter equal to the diameter of portion 10, is threaded externally and ends at its lower end in a tapering point 13. 
     Central portion 11 constitutes the fundamental characteristic of the invention. 
     The lateral surface of said central portion 11 has a cutting member 16 which extends parallel to the longitudinal axis of the bolt along the entire length of portion 11. The ends of the lateral surface of said portion 11 merge into the lateral surfaces of preceding portion 10 and following portion 12 so as to avoid forming sharp edges which could harm anyone handling the bolt too casually, in addition to causing problems when the bolt is positioned in the patient&#39;s bone. 
     In a first embodiment illustrated in FIGS. 1 and 2, said central portion 11 has a substantially drop-shaped transverse cross-section and has a lateral surface consisting of a semicylindrical portion 18 which is a continuation of the corresponding part of portion 10, while the remainder, having the same plane of symmetry as said portion 18, has two portions 20, which are externally convex, connecting said portion 18 with the lateral surfaces 21 of cutting member 16. Said cylindrical surfaces 21, having an externally directed concavity, by means of their line of intersection give rise to edge 15 of said member 16, lying along the axis of symmetry of the said section, at a distance from the longitudinal axis of the bolt equal to the radius of said portion 18. In this way said edge 15 forms a prolongation of a generatrix of upper portion 10 and in turn extends into a generatrix of portion 12. 
     A second embodiment of the invention provides a central portion 11&#39; having a transverse cross-section which is again substantially in the form of a drop as illustrated in FIG. 3. 
     Said portion 11&#39; has a symmetrical lateral surface with respect to a plane passing through the longitudinal axis of the bolt consisting of a cylindrical portion 18&#39; with a radius of curvature slightly greater than the radius of cylindrical portion 10, having its generatrix lying in the same plane of symmetry, as an extension to the generatrix of said portion 10, while the remaining portion has two portions 20&#39; of variable curvature, which are externally convex, followed by two other portions 22, which are internally convex, joining said portion 18&#39; with the lateral surfaces 21&#39; of cutting member 16&#39;. 
     By their intersection said flat surfaces 21&#39; determine the edge 15&#39; of said member 16&#39; lying along the axis of symmetry of the said portion, at a distance from the longitudinal axis of the bolt equal to the radius of said portion 10. 
     FIG. 4 illustrates the transverse cross-section of central portion 11&#34; of a third embodiment of the invention. 
     Central portion 11&#34; has a transverse cross-section which is still substantially in the shape of a drop and has a lateral surface which is symmetrical with respect to a plane passing through the longitudinal axis of the bolt. The said lateral surface consists of a cylindrical portion 18&#34; having a radius of curvature slightly less than the radius of cylindrical portion 10, the generatrix of which lies in the same plane of symmetry and is the extension of a generatrix of said portion 10, while the remaining portion consists of the two lateral surfaces 21&#34; of cutting member 16&#34;. Through the line of their intersection said surfaces 21&#34; give rise to the edge 15&#34; of said member 16&#34;. Said edge 15&#34; which lies in the plane of symmetry of the cross-section runs parallel to the longitudinal axis of the bolt at a distance from it of slightly more than the radius of cylindrical portion 10. 
     Clearly in order to limit the danger of cutting persons who come into accidental contact with such bolts when they are being handled said edge 15&#34; is merged into the surfaces of portions 10 and 12 at the two ends of portion 11&#34; so as not to leave sharp edges. 
     Even though not illustrated in the drawings, it is clear that portion 12 may be tapering or cylindrical, and may be provided with any known thread, either tapering or cylindrical. It is also clear that end 14 of the bolt according to the invention may also be shaped in a different manner from that illustrated provided that the said bolt can be engaged by a suitable instrument for tightening or slackening it. 
     Finally, it is clear that point 13 of the bolt may also not be pointed as illustrated in FIG. 1, but taper in a different way.